1. Description of the Related Art
The use of hollow threaded perforated interbody spinal fusion implants such as taught by U.S. Pat. No. 5,015,247 to Michelson (""247), incorporated by reference herein, is now commonplace. Nevertheless, because of the structure and biomechanical properties of these implants, the use of such implants has not been available for all patients requiring spinal fusion, but rather has been limited to a subset of that population. While such implants have proven to be very successful when used correctly, such success has not been universal.
A previously identified problem as discussed in U.S. Pat. No. 4,593,409 to Michelson (""409), incorporated by reference herein, is the frequent need for such implants to have a reduced combined width relative to their combined height. This permits the height, which is usually the implant diameter, to be sufficiently great so as to span the height of the distracted disc space and adequately penetrate and engage each of the vertebral bodies adjacent that disc space, and yet have a significantly lesser width so that when such implants are utilized in side-by-side pairs, the combined width is such that the paired implants do not protrude beyond the width of the spine. Historically, this not infrequent situation has deprived many patients needing spinal fusion from use of the prior art technology as implants of the desired height could not safely be placed within that patient""s disc space because of the width problem. Alternatively, downsized versions of these implants were implanted with poor results as the implants were of insufficient size to adequately function for their intended purpose.
As discussed in Michelson ""409, implants having various vertebral bone engaging surface projections have the advantage of enhanced stability within the spine as compared to an implant having a smooth surface. The use of a thread or thread portions has proven particularly beneficial and have been described in Michelson ""247. As described in Michelson""s co-pending application Ser. No. 08/484,928; 08/480,904;now U.S. Pat. Nos. 6,210,412 and 08/480,908 incorporated by reference herein, similar devices in which opposite vertebrae engaging arcuate surfaces are in angular relationship to each other may be useful to be fuse the vertebrae in a more lordotic angular relationship relative to each other. Implants of the related art are taller near the end adapted to be placed proximate the anterior aspect of the vertebral bodies than at the opposite end adapted to be placed proximate the posterior aspect of vertebral bodies. The related art implants are generally wedge-shaped when viewed from the side. The wedged configuration causes the implant to be less stable within the spine than if it were non-wedged. Further limiting the stability of these implants, compromising the surface area available for contact and fusion, and limiting the volume of osteogenic material containable within the implants is a result of the fact that these implants have generally been relatively flat across their trailing ends so as to be rotationally symmetrical about their mid-longitudinal axes. The anterior aspects of the vertebral bodies are generally curved from side-to-side. As a result, related art implants needed to be rather deeply inset into the disc space and away from the anterior aspects of the vertebral bodies so as to prevent the implants from protruding from the disc space at their lateral wall and trailing end junctions, wheresuch a protrusion of the implant could place vital structures adjacent the spine at risk.
There is, therefore, a need for further improvement in the design of such interbody spinal fusion implants so as to firstly extend their range of usefulness, and secondly to further increase the rate of success when such implants are used.
In accordance with the present invention, as embodied and broadly described herein, there are provided interbody spinal fusion implants that are threaded at least in part and require an element of rotation for insertion across a disc space between two adjacent vertebral bodies of a spine. The implants of the present invention are configured to be positioned in close proximity to each other such that the combined width of the implants is less than the combined height of the implants. The implants preferably have a leading end, a trailing end opposite the leading end, and a mid-longitudinal axis and length therebetween. The implants preferably have opposite arcuate portions adapted for placement toward and at least in part within the adjacent vertebral bodies and have a distance therebetween defining an implant height greater than the normal height of the disc space to be fused. Each of the opposite arcuate portions preferably has at least one opening in communication with each other for permitting for the growth of bone from vertebral body to adjacent vertebral body through the implant. Preferably, at least a portion of a thread is formed on the exterior of each of the opposite arcuate portions for penetrably engaging the adjacent vertebral bodies and to facilitate securing the implant into the spine by at least in part rotating the implant about its mid-longitudinal axis. At least a first one of the implants preferably has a lateral side wall and a medial side wall with a distance therebetween defining an implant width transverse to the implant height. The width of the first implant is less than its height along at least a portion of its length. The medial side wall of the first implant is preferably configured to be positioned in close proximity to at least a second spinal implant such that the combined width of the first and second implants is less than the combined height of those implants.
The present invention provides for improved interbody spinal fusion implants for placement within the spine in longitudinal side-by-side nested pairs. As used herein, the terms xe2x80x9cnesting or nestedxe2x80x9d refer to the placement of at least two implants in side-by-side relationship and close proximity to each other. In a preferred embodiment, the present invention teaches the nesting together of a pair of tapered root threaded spinal fusion implants, such that the nested implant pair has a combined reduced width relative to the combined height of the individual implants. As used herein, the terms xe2x80x9ctapered rootxe2x80x9d refers to an implant having an outer diameter as measured at the peaks of the bone penetrating protrusions, such as threads, and a root diameter, wherein the root diameter tapers from one end to the other end of the implant.
An embodiment of the present invention includes an interbody spinal fusion implant adapted to receive along its length a second circumferentially threaded complimentary interbody spinal fusion implant, such that the second implant nests within the circumference of the first implant. The nested longitudinal side-by-side pair has a combined width less than the implants"" combined maximum diameters, which maximum diameters generally define the over-all implant heights. In a further embodiment, the implants of the present invention are angled toward each other such that the combined width at the leading ends is further lessened.
The present invention implants have opposite arcuate portions and preferably are rotated into place. In an embodiment of the present invention, the implants may be generally cylindrical and have a thread or thread portions. In a preferred embodiment, the root diameter of the implant is generally conical or a portion of cone in that the opposite arcuate surfaces for contacting the vertebrae adjacent the disc space are in angular relationship to each other generally over the length of the implants. In a preferred embodiment of a tapered root implant, the outer diameter of the implant as measured at the thread peaks remains relatively constant over the length of most of the implant. As the root diameter of the implant tapers down, the thread height increases such that the outer diameter of the implant as measured at the thread peaks remains relatively constant.
The present invention has at least one of a pair of implants having at least one side adapted to receive within the over-all circumference of the outer diameter of the implant the side of a second implant. In a preferred embodiment, the receiving implant has both leading and trailing support walls, and while preferable, but not requisite, these walls may provide structural support and nevertheless be highly perforated to allow for vascular access and the growth of bone through the implant. In a preferred embodiment of a second implant to be received within the first implant, the leading end support structure further comprises a cap, which cap need not, but may be threaded, and which cap need not be, but preferably is perforated. In a preferred embodiment of the present invention, the trailing ends of the implants are rotationally asymmetrical about the mid-longitudinal axis such that they may be inserted in nested fashion and in proper rotational alignment relative to each other and to the vertebral bodies, with the result that the implants will have a length along the lateral aspect from leading to trailing end less than the length along the medial wall from leading end to trailing end. Preferably, the implants of the present invention are structurally adapted, such that when properly inserted, the length of the lateral side wall as measured from the leading end to the trailing end is of a lesser length than the length of the implant along its mid-longitudinal axis so as to prevent the protrusion of the lateral side wall and trailing end junction beyond the circumferentially curved profile of the vertebral bodies.
While the present invention does not require it, in a preferred embodiment the implants are adapted to receive through their trailing ends opposed bone screws and to transmit at least threaded portions of those screws through the opposite vertebrae engaging arcuate portions so as to allow those bone screws to engage at least one each into each of the vertebral bodies adjacent a disc space into which the implant are implanted.
Each of the embodiments of the implants of the present invention may also include one or more of anatomically contoured trailing ends, tapered minor diameters, opposed bone engaging screws, and locks for locking the opposed bone engaging screws into place. In a preferred embodiment, the implants of the present invention are configured to receive bone screw locks to lock the opposed bone screws to the implants. The bone screws are preferably lag screws and the locks, while preventing the backing out of the lag screws from the implant, may either be rigidly fixed or allow for continuing angular motion of the lag screws relative to the implants.
The present invention also is directed to an improved method for inserting such implants.
The accompanying drawings, which are incorporated in and constitute a part of this specification, are by way of example only and not limitation, and illustrate several embodiments of the invention, which together with the description, serve to explain the principles of the invention.